Should You Pay Off Your Mortgage or Invest?

The short version

Extra mortgage payments earn a guaranteed return equal to your mortgage rate — 6.9% here, risk-free. Investing the same dollars might earn more, but that return is an expectation with a wide range, not a promise. At 6.9% vs 7%, the guarantee wins by $159,862 over 28 years. Drop the mortgage rate to 3.5% and investing wins by $106,896; raise it to 8% and the payoff wins by $278,167. The rate is the whole debate — run your own numbers in the pay off mortgage vs invest calculator.

The verdict, line by line

Setup: $300,000 balance, 6.9% fixed APR, 28 years remaining, $500/month extra, 7% expected market return. Both paths spend identical monthly dollars, so the gap at the end is a fair verdict.

Line 1 — the scheduled payment: $300,000 at 6.9% over 336 months costs $2,019.11/month before extras.

Line 2 — the do-nothing cost: $2,019.11 × 336 − $300,000 = $378,419.98 of lifetime interest. That number is the prize the payoff path is hunting.

Line 3 — the payoff path: paying $2,519.11/month kills the loan in 202 months (16 years 10 months). Interest falls to $207,221.58, so $171,198.40 is erased and 134 months (11 years 2 months) are shaved off. The freed $2,519.11/month then compounds at 7% for the remaining 134 months, growing to $509,645.99.

Line 4 — the invest path: $500/month at 7% for the full 336 months grows to $519,344.11 — a bigger account, but the mortgage still costs the full $378,419.98 in interest.

Line 5 — the verdict: payoff path nets +$786 after all cash paid; invest path nets −$159,076. Paying off wins by $159,862. The 7% sounds better than 6.9%, but it applies to $500 a month while the 6.9% applies to a $300,000 balance — and the guarantee needs no luck.

Branch 1: the $50,000 lump sum

A bonus or inheritance applied to principal — no monthly extra — retires the loan in 218 months (18 years 2 months), saves $190,118.69 in interest, and shaves 118 months off the schedule. It saves more than $500/month does ($171,198), because the lump lands in month one, where interest is thickest. Early dollars are the most expensive dollars on an amortization schedule: a dollar of principal erased in year one deletes up to 28 years of interest charges on that dollar.

Branch 2: the 3.5% loan — investing wins clearly

Same $300,000 and $500/month, but a 3.5% rate: the payoff saves only $68,569 in interest (done in 212 months), while the $500/month compounding at 7% builds the bigger position. Investing wins by $106,896. A guaranteed 3.5% can’t keep up with a 7% market expectation. If you locked a pandemic-era rate, minimum payments plus aggressive investing is the comfortable answer — stop feeling guilty about not prepaying.

Branch 3: the 8% loan — payoff wins by a mile

At 8%, extra payments earn an 8% risk-free return no bond fund will offer. The loan dies in 197 months (16 years 5 months), $212,974 of interest is erased, and the payoff path wins by $278,167. High-rate debt is an emergency wearing a suit: after the 401(k) match (below), prepay it before investing a discretionary dollar.

Branch 4: moving in 7 years — a coin flip

Short horizons compress the debate. Over 7 years, the extra $500/month builds $53,799 of additional equity; investing it builds a $53,999 account — a $200 difference. When the horizon is short, liquidity decides, not math: a brokerage account is spendable tomorrow, while equity comes back only at closing. If you might move soon, keep the money liquid unless your rate is very high.

Branch 5: the 401(k) match comes before both

None of the above applies until you’ve taken the employer match. A typical 50% match turns $500/month into $750/month instantly — $3,000 a year of free money, an instant 50% return no prepayment math and no market can beat. Fund the match first, then compare payoff vs invest with what’s left. Skipping the match to prepay is leaving a winning lottery ticket on the table.

The rate-vs-return rule of thumb

Skip the calculator with this: subtract your mortgage rate from the market return you honestly expect. Wide gap in the market’s favor (3.5% vs 7%) → invest. Narrow or negative (8% vs 7%) → prepay. Near the boundary (6.9% vs 7%), the guarantee’s edge — no volatility, no sequence risk, no bad decade — tips it to payoff. And haircut the market return for reality: 7% is a long-run average across decades, not a promise for your decade. If you’d invest conservatively at 4–5%, the payoff path wins by even more.

Liquidity: the cost the math doesn’t show

The $159,862 verdict has a blind spot: extra principal is trapped. It becomes equity, and equity is illiquid — reaching it means selling, refinancing, or a HELOC, all slow and sometimes expensive. An investment account can be tapped in days. So the order is: emergency fund, then 401(k) match, then high-rate debt, then the payoff-vs-invest question. Never prepay with a thin cash cushion or while carrying 20%+ card debt. Model the liquidity side in the emergency fund vs invest calculator.

The mortgage-interest-deduction note

Itemizers get a discount on prepaying: deducting interest at a 24% marginal rate turns 6.9% into an effective ~5.24% after-tax cost, weakening the payoff case. Three caveats. One: since the standard deduction rose, most borrowers don’t itemize — no itemizing, no deduction, no adjustment. Two: the deduction only applies to interest you actually pay, and prepaying erases the interest and the deduction together. Three: it’s worth your marginal rate times the interest, not the interest itself — it trims the payoff return, never flips it. If you itemize, shave a quarter off your rate and re-run the calculator.

Three mistakes that wreck the verdict

Mistake one: comparing the wrong returns. People benchmark their mortgage rate against last year’s stock returns or a friend’s crypto story. Compare it to the return you expect from the portfolio you’d actually hold — then give the guarantee its due for being guaranteed.

Mistake two: prepaying while illiquid. Extra payments feel productive, so people prepay money they’ll need in six months — then borrow it back at 24%. The $159,862 win assumes the dollars stay put for 28 years. If they won’t, the invest path’s liquidity is worth more than the spreadsheet says.

Mistake three: skipping the match and the expensive debt. Prepaying a 6.9% mortgage while skipping a 50% 401(k) match, or while carrying 22% card balances, is arithmetic malpractice. The sequence is match, emergency fund, 20%+ debt, then payoff vs invest — get it right and this guide’s verdict actually applies to you.

The bottom line: at 6.9% the guaranteed return is genuinely excellent — $171,198 of interest erased, 11 years 2 months shaved, a $159,862 win over investing at 7%. At 3.5% the market wins; at 8% the payoff wins by a mile; over 7 years it’s a tie. Take the match first, keep your cushion liquid, then let your rate decide. Run your exact numbers in the pay off mortgage vs invest calculator.